Roy G. Biv, dude. A rainbow has seven colours; Mist's point is that both the LGBTQ rainbow flag and AlphaCool's promotional imagery have only six colours, and the colour they're both missing is indigo. A coincidence, or are The Gays trying to infect our rigs with fabulousness?

(Spoiler: it's probably 'cos indigo dye is difficult to distinguish from violet so often gets left out of fabric rainbows.)

"It is customary to list indigo as a color lying between blue and violet, but it has never seemed to me that indigo is worth the dignity of being considered a separate color. To my eyes it seems merely deep blue."

According to Asimov, indigo isn't really a color. I trust him, because he worked out how to stop robots from killing us all.